This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The present invention generally concerns a method for starting large rotating equipment. More specifically, it concerns a compressor starting torque converter for a string of equipment. The starter is required for initiating a large motor or turbine to drive a compressor or a multiplicity of compressors. The compressor string may be useful in large scale liquefied natural gas (LNG) refrigeration.
Combinations of high speed, high power rotating equipment (e.g. turbines, electric motors, and compressors) in serial combination (a.k.a. “strings”) generally require separate starters to initiate operation due to numerous factors such as: the large moment of inertia, parasitic losses, break-away torque, windage, compression load, and other resistance associated with the rotating equipment. A typical string in a facility may have a gas turbine or motor driver connected to a compressor, multiplicity of compressors, generator, or any another rotating machinery based load. A starter mechanism, such as a low power starting motor, may also be connected to the string. FIG. 1 shows an example of a typical string with a gas turbine 3 mechanical drive and compression load, including a first compressor 4 and a second compressor 5, with a variable frequency drive (VFD) 1 with starter motor (S/M) 2. The VFD 1 is an electric device that inverts fixed alternating current (AC) line input voltage to direct current (DC) and converts DC voltage to user defined output AC. The VFD 1 produces a user selectable variable frequency output, thereby providing variable speed for the S/M 2. As a result, large inertial loads are started with limited controlled in-rush current as opposed to across-the-line starts in synchronous motors with damper bars which may draw up to six times (load dependent) the motor rated current for continuous duty operation.
FIG. 2 schematically illustrates a typical across-the-line starter motor (S/M) 6 with a variable speed hydraulic clutch (HC) 7. The across-the-line (S/M) 6 with HC 7 is another mechanism for starting large inertial loads. The HC 7 operates as a mechanical VFD. The across-the-line S/M 6 is started with the HC 7 disengaged and thus with no load. Once the S/M 6 is at full speed, the HC 7 is engaged, providing variable speed (zero to full speed) and necessary torque to bring the gas turbine 3 and other connected load(s) (e.g. a first and second compressor 4, 5) to full speed. Once the gas turbine 3 achieves sufficient speed to produce power, the HC 7 is disengaged and the S/M 6 is electrically removed from service.
Starting a string may be achieved under one of two primary conditions. The first condition is a depressurized start, and the second is a pressurized start. A depressurized start initiates at a low settle-out pressure within the compressor(s). For a depressurized start, the working gas is removed from the compressor(s). The working gas is replaced, the method for replacement being known as gas make-up. Gas make-up may require extra facility hardware (valves, piping, transmitters, flares, gas reclamation, and associated controls) and is a time consuming effort. Due to the lengthy time requirement and extra facility cost to make-up gas in a depressurized start, a pressurized start is an attractive alternative. A pressurized start initiates with a high settle-out pressure within the compressor(s) compared to depressurized start. A pressurized start removes necessary hardware associated with the gas make-up of depressurized start; however, it requires additional starting power due to a higher starting torque necessitated by gas in the compressor causing a higher internal compressor load.
There are generally two types of turbines, dual-shaft and single-shaft. Dual-shaft gas turbines produce compression for the combustion process with a compressor driven by a low count (e.g. 2 or 3) stage turbine on a single shaft. The remaining thermodynamic power in form of pressure, temperature, and mass flow is routed directly into the coaxial power turbine on a second shaft. An advantage of a dual-shaft gas turbine is the ability to produce significant power (torque) across the turbine's speed range. However, as compressor string power requirements have increased, the demand for larger power gas turbines has also increased. To meet these power requirements, users have adapted single shaft gas turbine technology traditionally used in power generation for mechanical drive service.
For very large motors or turbines under a pressurized start, an across the line starter motor, as shown in FIG. 2, is insufficient. Accordingly, variable frequency drives (VFD) provide the startup power. However, a VFD is a large capital expense and may contribute up to 70% of the cost in a motor/VFD package. A mechanical alternative to starting a compression string is the variable speed fluid drive. A variable speed fluid drive is a constant speed input, variable speed output device that transmits power (torque) from the input shaft to the output shaft via a fluid (hydraulic) coupling. A more common name for a variable speed fluid drive is a compressor starting torque converter (CSTC). A CSTC generally includes a synchronous speed lockup feature, which allows the drive end and the driven end of the CSTC to mechanically engage and enable rigid rotation between the prime mover and the load(s) at synchronized speed.
FIG. 3 depicts another known system comprising a prime mover 10 (e.g. gas turbine or electric motor) with a CSTC 12. The CSTC 12 eliminates the need for a VFD in a motor driven string and eliminates the need for a starter/helper motor and VFD in a gas turbine driven string. However, a S/M 6 with a HC 7 or starter motor VFD is required in either case to start the string. The starting package of a prime mover 10 (e.g., gas turbine or motor) can be either a S/M 6 with HC 7 or a starter motor with starter motor VFD. Either starting package may be interchangeably used with any prime mover type, such as, for example, a gas turbine or motor. U.S. Pat. No. 6,463,740, issued Oct. 15, 2002 to Schmidt et al. is an example of a CSTC in a string. The CSTC of Schmidt is coupled to a gas turbine and a compressor, but is incapable of starting up a compressor string of over 50,000 horsepower (hp) (less than 40 megawatts (MW)).
Power to speed performance of the CSTC is limited. Certain applications require more power at a given speed than CSTC technology can presently support. In a depressurized start, where the loads in a compressor string are reduced, a CSTC may be capable of performing a start; however, CSTC technology alone is power-limited in starting high load compressor strings such as pressurized starts in high-load compressor strings. The CSTC power output decreases as its speed increases. Given the high speed requirement of current high power compressors, the CSTC is not capable of outputs at the necessary speed and power. The present invention seeks to mitigate this shortcoming.
Other related material may be found in at least U.S. Pat. Nos. 2,377,851; 3,043,162; 3,886,729; and 3,955,365; and G.B. Pat. No. 1,208,831.